The present invention relates to a method of sealing a semiconductor device with resin, and more particularly to a method of resin molding a device having a heat sink or thermal radiation board.
Conventionally, sealing a semiconductor device having a heat sink with resin has been carried out in the following manner.
First, glass powder is deposited on the upper surface of a heat sink, and a lead frame is put on the glass powder and then these are heated to a temperature of about 350.degree. C. The melted glass powder bonds together the lead frame and heat sink in an electrically insulating state. A semiconductor chip is die-bonded on the upper surface of an island which is located at the center of the lead frame. Then, pads of the semiconductor chip and the tips of inner leads of the lead frame are bonded together by means of wires.
Next, the heat sink together with the semiconductor chip and lead frame is placed on the bottom surface of a lower mold within its cavity, and an upper mold is mounted on the lower mold to enclose the cavity defined by the upper and lower molds.
Melted resin is injected by a transfer molding method into the cavity defined by the upper and lower molds. After the resin is hardened, the resin sealed semiconductor is picked up by removing the upper and lower molds.
With the above resin sealing method, however, melted resin is injected within the cavity defined by the upper and lower molds, and some of melted resin may flow into a small gap between the heat sink and the bottom surface of the lower mold. Therefore, the bottom surface of the heat sink of a finished semiconductor device is often covered with resin which obstructs the heat dissipation from the surface of the heat sink. To avoid this, there has been proposed a method wherein a peripheral portion of the heat sink is projected downward and is made in tight contact with the bottom surface of the lower mold to thereby prevent melted resin from flowing toward the bottom of the heat sink. It has been found, however, that this method cannot fully prevent such resin flow.
Further, the lead frame may sometimes deformed (bent or warped) while resin is injected so that there is associated with a problem of poor workability in mounting a semiconductor device on a printed wiring board.